Step 1: Social contract theory.
Social contract theory explains the origin of political authority through agreements among individuals.
Step 2: Two foundational pacts.
{Pactum unionis} refers to individuals uniting to form a society, while {pactum subjectionis} refers to submission to a common authority.
Step 3: Conclusion.
Both pacts together form the basis of the social contract. Hence, option (C) is correct.